These are things I've noticed over the years, both as a writer and a reader.
Not having a web site or blog that is up to date with what's coming, what available and where to find them. This is a critical mistake these days. The first thing many readers do when they hear about an unfamiliar author is search Amazon for it. Then, if they're like me, they Google that author, looking for more information about her. If I don't find a blog or web site, I will often go on to another one. The only reason I wouldn't at this point, is if the author or book was recommended by a trustworthy friend. What reason would an author today, unless he's already got instant name recognition, not have either a web site a blog, or both?
Recently I came across an excerpt from a novel that looked intriguing, so I looked for the buy link. There was none. The author mentioned the publisher, but the only links were to another publisher. Maybe she had books there too, but what about the book she was trying to sell with that excerpt? Making a potential reader search for what it is you want them to see is a sales killer. I actually went further than I normally do. In most cases I simply go somewhere else. On line these days people have no patience. They want it now. If you don't give them that, they'll go to another author who has it all laid out, in her email, on her web site and blog. Facebook and Twitter too should contain information about you. Facebook has an iinfo tab you can set up with links to all your sites. Not to do so is not only lazy, it's self destructive.
These days many agents and editors will often check an author whose query interest them out online. This shows the agent not only that you are actively prepared to promote your work, but what the tone of your writing is. An author who is negative, attacks other authors, publishers or agents might turn on alarm bells and make them think twice about taking that author on as a client.
Published authors also need to take care in their posts, or responses to other posts. I'm sure we all remember the recent melt down by an author with her online attack on a reviewer. You need to avoid showing bad temper or poor judgment. The publishing world is very small, and especially for the ones who live in New York, they eat and drink together. Others meet at conferences. It's all good networking and like any other profession, they like talking about their job. Or maybe more likely, complaining about some of the negative things they have to deal with. I know that's something I've always done with co workers. It's a good way to vent. But the last thing you want is for your name to come up in a conversation along the lines of "You won't believe what happened today." and your name follows. Why would someone want to start a long term professional relationship with a writer who is confrontational or hard to work with?
We all have to believe our writing is good, how else can we keep butting our head against the brick wall of rejection? But believing in our work does not mean we can attack people who don't share our love. Sometimes this means you have to bite your tongue and walk away. The most you should ever do is respond positively. If it's a negative review, say thank you and let it go. If it's a criticism from a critique partner or beta reader, again, smile and say thanks, I'll look at it.
It all sounds like something that would seem like common sense. But when it comes to common sense it can go by the wayside when your baby is criticized.
So two things you need to increase your professional chances: make yourself and your work easy to find and be polite to everyone and never forget this is a business. Treat it that way and you will be rewarded by professionalism in return.